This invention relates to a mechanism for controlling momentum of atoms, molecules or ions which exhibit quantum-level reactions to the quantum stimulation of electromagnetic energy, hereinafter referred to as quantum objects. Specifically, this invention relates to manipulation of internal states of individual quantum objects typically using coherent electromagnetic energy, and more particularly this invention relates to controlled manipulation of individual atoms using stimulated Raman transitions. Specifically, this invention relates to use of stimulated Raman transitions to excite individual atoms to an excited internal energy state which is metastable or substantially non-radiation emissive over time periods of interest.
The principle of stimulated Raman transitions has been known for decades. Stimulated Raman transitions (SRTs) are for example the quantum excitation of an atom using electromagnetic waves of two different frequencies which are mixed (or, in the specific case of only two frequencies, hetereodyned) (i.e., in the same region of space at the same time) to induce a change of internal energy state of the atom. Stimulated Raman transitions can be induced in any quantum object.
An uncontrolled recoil effect has been observed in the course of research on stimulated Raman transitions. Heretofore, the recoil effect has not been harnessed and made useful. What is needed is a technique for controlling the observed recoil effect to do useful physical work.
The Doppler effect is a well-known phenomenon of physics in which the process of reflection causes an observable change in frequency in a reflected wave of radiation due to relative motion. The process of reflection changes the frequency of the radiation according to the formula .DELTA..nu./.nu.=2v/c, where .nu. is the frequency of the radiation, .DELTA..nu. the frequency change, v the velocity of the object and c the speed of light. A sensitive frequency difference detector, such as a heterodyne detector, may be used to measure the frequency change .DELTA..nu., so that if the initial frequency .nu. is known, the velocity of the object of reflection (i.e., a "mirror") can be determined from the above formula.
This technique, however, does not work well when the mirrors are atoms. There are two problems: an atom is a poor mirror in that it does not reflect a substantial fraction of the incident light, and the light itself exerts forces on the atom which can substantially change the velocity of the atom.
It is known that the properties of an atom (the internal state of the atom) are altered when an atom scatters incident radiation. One aspect of the interaction of light with an atom is that the atom will only absorb or scatter (reflect) light for certain well defined frequencies (called resonances). Furthermore, under certain circumstances, the absorption or emission of the radiation will be accompanied by a change in the internal state of the atom which persists long after the excitation from radiation is ended, herein referred to as a metastable state.
In the case of electromagnetic excitation of quantum objects, such as atoms, ions or molecules, the Doppler effect manifests itself in a more subtle manner: the quantum object will only change its internal state when the velocity of the quantum object is such that the radiation frequency is Doppler shifted to a frequency which resonates with internal quantum levels of the quantum object. Assuming a technique exists to determine selected quantum level states of the target quantum object, the velocity of the quantum object can be measured by changing the frequency of the incident radiation until the quantum object changes its internal quantum state. The velocity could then be determined by comparing this frequency with the known resonance frequency for a quantum object such as an atom at rest, i.e., the velocity would be given by v/c=.DELTA..nu./.nu., where .DELTA..nu. now refers to difference between the resonance frequency for a moving atom and the resonance frequency for an atom at rest. This technique has been used in the past, for example, to study atomic collisions, but heretofore the principles have never been controlled and applied to perform useful work.
The accuracy of measuring the velocity of an atom, as a typical quantum object, with a Doppler technique is limited by the accuracy of the measurement of the frequency shift .DELTA..nu.. The accuracy of this measurement, in turn, is limited by a well known result of the theory of Fourier transforms, namely that (.delta..nu.)t.about.1, where .DELTA..nu. is the uncertainty in the frequency measurement and t is the time of the measurement. More accuracy requires longer measurement times. On the other hand, for a fixed time t, accuracy in measurement of v is enhanced by making the probing radiation frequency .nu. as large as possible. For an atom .nu. can be in the visible portion of the electro-magnetic spectrum (10.sup.14 -10.sup.15 Hz). However, there are two problems associated with the use of optical frequencies. First, the excitation-induced internal quantum state associated with the resonance typically decays back to the atom's original internal quantum state (ground state) in time scales on the order of 10-100 nanoseconds (the lifetime of the resonance). This decay time sets on upper limit on the measurement time t which in turn limits the velocity resolution of the technique. However, atoms possess `metastable` resonances whose decay times are abnormally long (.about.milliseconds). Unfortunately, to make use of the metastable resonances of atoms in particular a light source with a well-defined frequency is required. These are difficult to build, and have absolute stabilities which are presently limited to about 50 Hz, which is not as stable as desired for high-accuracy measurement. What is therefore further needed is a mechanism for improving the accuracy of measurement of Doppler effects on the scale of atoms and the like.
Multiple-pulse sequence excitation is known in the field of optical Ramsey spectroscopy, a superset of stimulated Raman spectroscopy. Optical Ramsey spectroscopy uses coherent superpositions of quantum states to measure frequency of optical radiation. In addition, it is known to use multiple-pulse excitation in connection with the Doppler effect to measure rotations, analogous to the measurements made by a gyroscope. Much of the art concerning the multiple-pulse sequences stems from an alternative view of the coherent interaction of the light with atoms. See Borde, C. J., Physics Letters A, Volume 140, page 10, (1989). This view stresses the momentum exchange which occurs during the interaction and leads to what is called matter-wave interference. Therein the atom is viewed as a quantum mechanical wave with properties analogous to the wave properties of light. (The characteristic wavelength used to describe a matter-wave is the deBroglie wavelength .lambda..sub.dB =h/mv, where h is Planck's constant and v is the velocity of the atom.) In that approach, matter-wave interference effects lead to the changes in the probabilities of finding an atom in particular internal state. The basic ideas of the Borde approach are summarized in the following paragraphs in order to distinguish the present invention.
In addition to changing the internal state of the atom, electromagnetic pulses when absorbed in an atom also change the velocity of the atom. The physical reason is conservation of momentum: the total momentum of the combined system consisting of the atom and the laser light field must be conserved during interaction of the atom with the light. Consequently, when an atom absorbs a photon of momentum hk (the photon is the quantum mechanical particle which is used to describe the light field), its velocity must change by .DELTA.v=hk/m, where m is the mass of the atom, h is Planck's constant, and k=2.pi./.lambda. is the propagation vector of the light. This is in addition to the change in the internal quantum state of the atom which occurs as a result of the absorption of the photon. Similar theories explain general stimulated emission processes: in making a transition to an internal state of lower energy than the initial internal state, the velocity of an atom also changes as a result of conservation of momentum.
For the multiple-pulse sequences of optical Ramsey spectroscopy (e.g., those referred to in the literature as the sequences having the quantum description .pi./2-.pi.-.pi./2, as explained below), the correlation of the internal state of the atom with its velocity leads to a physical splitting and recombination of the atom. (Consider an atom with internal states 1&gt; and 2&gt;, as denoted by standard quantum state notation. A ".pi./2" light pulse is one which puts the atom in the superposition state 1/sqrt(2) ( 1&gt;+2&gt;). A ".pi." light pulse takes an atom in the 1&gt; quantum state and drives it to the 2&gt; quantum state. In general, a pulse of area "a.pi." (where a is a positive real number) puts the atom in the superposition state .alpha..sup.2 1&gt;+.beta..sup.2 2&gt;, with .alpha..sup.2 +.beta..sup.2 =1. The values of .alpha. and .beta. depend on the pulse area. The pulse area depends on the duration of the light pulse, the intensity of the light, the frequency of the light and the velocity of the atom.) It should be understood that associated with each type of pulse sequence is a delay or a space between pulses. For example, the delays in a sequence such as .pi./2-.pi.-.pi./2 are substantially equal in duration. Although the delays may in theory be zero, it is preferable that the delays are equal and as great as practical. A reasonable maximum duration of a pulse is 100 .mu.sec, if gravity is a factor. If gravity is not a factor, the duration is not limited. A reasonable maximum duration of a delay is 1000 msec. For other pulse sequences, the delays may be different. For the .pi./2-.pi./2-.pi./2 sequence, the relative delays between pulses are understood also to be equal at the beginning and at the end of the sequence, with the intermediate delay being arbitrarily long within practical limits.
The key components in any interferometer (optical or matter-wave as explained below) are a beam splitting and reflection process. The division and recombination of an atom described above is analogous to that which occurs in an optical Mach-Zehnder interferometer and is therefore useful in constructing a matter interferometer. In a Mach-Zehnder optical interferometer, a light beam (photon) is split in two at a first beam splitter. Mirrors subsequently redirect the two spatially separated paths of light. A final beam splitter recombines the light. The two recombined waves interfere constructively or destructively in the output arm of the interferometer. Analogously, in optical Ramsey spectroscopy, the first .pi./2 pulse serves as the matter-wave beam splitter: the part of the atom in internal quantum state 2&gt; takes a different spatial path than the part in internal quantum state 1&gt; as a result of the correlation of the atom's velocity (momentum) with its internal state. The .pi.-pulse serves as the mirrors: it "reflects" each part of the atom so that the two trajectories will again overlap. The final .pi./2 pulse recombines each half of the atom so that the matter waves subsequently interfere.
The potential utility of atom matter-wave interferometers as inertial sensors is known. In 1975, Collela, Overhauser, and Werner demonstrated that the acceleration of a neutron due to gravity could be measured with a matter-wave interferometer. The sensitivity of a matter-wave interferometer to inertial forces can greatly exceed that of a neutron interferometer, largely due to the increased mass of the atom and the availability of slow sources of atoms.
John F. Clauser of the University of California, Berkeley, has investigated the various atom interferometer geometries which utilize micro-fabricated material gratings or standing waves of light to divide and recombine an atom (see U.S. Pat. Nos. 4,874,942 issued Oct. 17, 1989 and 4,992,656 issued Feb. 12, 1991). David Pritchard of the Massachusetts Institute of Technology has experimentally demonstrated matter-wave interference in a geometry consisting of three successive micro-fabricated matter-gratings. (see D. W. Keith et al., Phys. Rev. Lett., Volume 66, page 2693, 1991). The geometry is the same as that used by Marton in 1954 to demonstrate matter-wave interference with electrons.
Interferometers using standing waves of light have also been proposed by Chebotayev (see V. P. Chebotayev et al., J. Opt. Sci. Am B, Vol. 2, page 1791 (1985)), and experimental observation of atom diffraction from standing waves has since been observed by Pritchard.
Altshuler and Franz, in U.S. Pat. No. 3,761,721, describe one of the first matter interferometers. Subsequent work in matter interferometry followed general principles described in the Altshuler et al. patent.
Borde demonstrated optical Ramsey fringes in the 70's and subsequently noted that the fringes can be interpreted as manifestations of matter-wave interference (Borde 1989, cited above). In the 1989 Borde work, an atomic beam is described which intersects four spatially separated travelling-wave laser beams oriented perpendicularly to the atomic beam.
The above interferometers which use light to manipulate the atoms do so in geometries where the transit time of the atom through spatially-isolated, independent beams of light, which are oriented perpendicularly to the atom's mean velocity, determines the effective time of interaction of the atom with the light. Furthermore, transit time between successive beams sets the time between interaction with the light.
The work describing the background to the present invention was first presented orally May 2-5, 1990 at Elba Island, Italy. No written descriptions of the background of the invention were made available at that time. An edited report on a proposed experiment to measure acceleration due to gravity using stimulated Raman transitions was first distributed by the publisher ETS Editrice Pisa about Jun. 1, 1991 under the title "The Production and Uses of Slow Atomic Beams," by D. S. Weiss, E. Riis, M. A. Kasevich, K. A. Moler and S. Chu, Light Induced Kinetic effects on Atoms, Ions and Molecules, Proceedings of the Workshop Held in Marciana Marina, Elba Island, Italy, May 2-5, 1990, ETS Editrice, Pisa, Italy. Experimental results were published under the title "Atomic Interferometry Using Stimulated Raman Transitions" in Physical Review Letters, Vol. 67, Jul. 8, 1991. This paper reported on work where the acceleration due to gravity was measured using Doppler shift accuracy of 3 parts in 10.sup.6. Subsequent improvements in the technique according to the background of the present invention have placed the experimental resolution at 3 parts in 10.sup.8.
A report of work on the background of the present invention entitled "Atomic Velocity Selection Using Stimulated Raman Transitions," Physical Review Letters. Vol. 66, No. 18,p. 2297 was published on May 6, 1991 under the authorship of the inventors and collaborators D. Weiss, S. Kasapi, E. Riis, and K. Moler. Therein is a report on a demonstrated preparation of an ensemble of atoms with a 300 .mu.m/sec velocity spread (corresponding to an effective one-dimensional temperature of 30 pico-Kelvins). A monograph in which the idea of using stimulated Raman transitions for controlling movement of atoms was reported by the inventors and their collaborators was first released about Aug. 1, 1991 under the title "Applications of Laser Cooling and Trapping," AIP Conference Proceedings 233, Zorn and Lewis, Editors, Atomic Physics 12 (Twelfth International Conference on Atomic Physics, Ann Arbor, Mich. 1990 (Copyright 1991 by the American Institute of Physics)). Although these publications appeared before the filing date of the present application, they do not describe the present invention.